1. Field of the Invention
The present invention relates to an echo canceller used to eliminate echo noises caused by acoustic coupling during communications between a telephone talker and a listener, for example, in a hand-free telephone system.
2. Description of the Related Art
For example, in a hand-free telephone system, echo noises caused by acoustic coupling between a microphone (transmitter) and a loudspeaker (receiver) have been a cause for the reduction in speech quality. The hand-free telephone system herein refers to a system which is one of forms of car phone systems predisposed at a specified position in a car to ensure operability and safety for a driver, thereby allowing a telephone call using a microphone and a loudspeaker.
An echo canceller has been used to eliminate such echo noises. The conventional echo canceller is equipped with an adaptive digital filter composed of a transversal filter. This adaptive digital filter estimates impulse responses, i.e., transmission characteristics of an echo path formed between a loudspeaker and a microphone. The transversal filter is designed to be activated imitating these transmission characteristics of the echo path. This imitation process is achieved by changing numbers of taps of the transversal filter. By inputting part of receiving signals to be fed to the loudspeaker into this transversal filter thus adapted to be activated imitating the transmission characteristics of the echo path, pseudo echoes being closely analogous to such echo noises as described above can be obtained from the output of the filter. Echo noises are thus eliminated by subtracting the pseudo echo signals from sending signals fed by the microphone.
In the related arts, however, there still exist problems to be resolved. That is, as described above, the pseudo echo is the output obtained by inputting part of receiving signals into the transversal filter. Therefore, the amplitude level of the pseudo echo is not allowed to exceed that of the receiving signals. However, in some cases, echo noises mixed with the sending signals may be signals the amplitude of which exceed that of the echo noise, which occurs, for example, when the volume of a microphone is made larger. In such a case, there is a problem left that the echo noise cannot be eliminated by using the pseudo echo of this kind.
The present invention has been made to solve the above problems and includes the following three constitutions.
One aspect of the present invention as shown in the Constitution 1 is to provide an echo canceller for eliminating echoes produced through an echo path formed between a loudspeaker used to convert receiving signals into voices to be emitted and a microphone used to convert voice inputs into sending signals, comprising an adaptive digital filter designed to be activated imitating transmission characteristics of the echo path and to generate pseudo echo signals using part of the receiving signals fed therein and to output them, an attenuator used to attenuate the sending signals containing echo noises fed by the microphone to which echoes have been inputted so that the amplitude level of the echo noises is matched to that of the pseudo echo signals and a subtracter used to subtract the pseudo echo signals from the sending signals containing the attenuated echo noises.
Another aspect of the present invention as shown in the Constitution 2 is to provide an echo canceller for eliminating echoes produced through an echo path formed between a loudspeaker used to convert receiving signals into voices to be emitted and a microphone used to convert voice inputs into sending signals, comprising an adaptive digital filter designed to be activated imitating transmission characteristics of the echo path and to generate pseudo echo signals using part of the receiving signals and output them, a subtracter used to subtract the pseudo echo signals from the sending signals containing the echo noises fed by the microphone to which echoes have been inputted and an amplifier used to amplify the pseudo echo signals so that the amplitude level of the pseudo echoes is matched to that of the echo noises and to feed the amplified signals to said substracter.
A third aspect of the present invention as shown in the Constitution 3 is to provide an echo canceller for eliminating echoes produced through an echo path formed between a loudspeaker used to convert receiving signals into voices to be emitted and a microphone used to convert voice inputs into sending signals, comprising an adaptive digital filter designed to be activated imitating transmission characteristics of the echo path and to generate pseudo echo signals using part of the receiving signals and output them, a subtracter used to subtract the pseudo echo signals from the sending signals containing the echo noises fed by the microphone to which echoes have been inputted and an amplifier used to amplify part of the receiving signals so that the amplitude level of the pseudo echoes is matched to that of the echo noises and to feed them to said adaptive digital filter.